There is a shock absorber piston known from EP-A-0 658 611 that has a piston body that is provided with a peripheral web at its peripheral surface. A number of webs running in the axial direction attach to this peripheral web on one side of it. A seal made of thermoplastic synthetic is sprayed onto this piston body in an injection molding process. The webs extending in the axial direction and the notches between them, which are filled in with sealing material, serve to reliably anchor the sealing material and to guide the piston. The seal applied by injection molding enables close tolerancing, which prevents xe2x80x9cblow-byxe2x80x9d and thus provides a reliable seal of the cylindrical spaces facing one another. The process to produce these types of injection-molded seals is relatively expensive.
There is a piston-cylinder arrangement known from U.S. Pat. No. 3,212,411 whose piston body has a number of peripheral grooves on its peripheral surface. To apply the seal, a cup-shaped preliminary mold made of PTFE (polytetrafluoroethylene) is provided that is first placed on the piston body loosely. The piston body so prepared is then pressed into a forming and calibrating cylinder that is heated to a high temperature. Under the influence of the heat, the PTFE material is pressed into the grooves on the peripheral surface of the piston body. Then, the piston body with the pressed-on seal is cooled in an appropriately designed cooling cylinder. The grooves are completely filled with the sealant material so as to provide a form-locked solid connection of the seal to the peripheral surface of the piston body. When used as a shock absorber piston, the bottom surface of the preliminary mold that still overlaps the end surface of the piston body on one side must then be removed.
There is a shock absorber piston known from EP-A-682 190 whose only essential difference from the processes described above in its manufacture is in that to apply the seal, instead of a cup-shaped preliminary mold, a stamped circular sleeve is used. This circular sleeve is placed on one end of the piston body. The piston body prepared thusly is then pressed into a heated forming and calibrating cylinder, wherein the circular sleeve is placed around the peripheral surface of the piston body as a strip and then pressed into the grooves running in the peripheral direction of the piston body under the influence of heat. Then, the piston with its pressed-on seal is guided through a cooling tube. Here, as well, the sealant material fills the grooves practically completely so that the seal is solidly connected to the peripheral surface of the piston body in form-locked fashion.
The two processes described above have the disadvantage in that considerable pressures are required to shape and to press the sealant material into the grooves on the peripheral surface of the piston body. Also, the sealant material forming the seal is subject to strong shaping forces that disadvantageously-influence the structure of the sealant material.
The objective of this invention is to produce a piston, in particular a shock absorber piston, in which the disadvantages described above are avoided.
This objective is met according to the invention by a piston for a piston-cylinder arrangement, in particular a shock absorber piston, with a piston body that is provided with at least one peripherally running web on its peripheral surface in an area adjacent to one end of the piston, said web protruding above the peripheral surface. Support webs extending to the other end of the piston running parallel in the longitudinal direction and separated from one another attach to this peripheral web. Between any two neighboring support webs, which are open in the longitudinal direction at their ends opposite the peripheral web, is a notch-shaped recess. The piston according to the invention also has a collar-shaped seal made of a thermoplastic sealing material that is formed onto the piston body in such away that both the peripheral web as well as the support webs press into the material of the seal at least over a portion of their height. Surprisingly, it has been shown that, in order to get a good seal between the collar-shaped seal of the piston on the one hand and the interior wall of the cylinder on the other, it is not required that the seal lie against the full height of the piston body. It is enough for the sealing collar to be supported in the peripheral direction along the relatively narrow peripheral web. Furthermore, it has been shown surprisingly that, for an acceptable and reliable connection between the seal and the piston body, it is not required that the notch-shaped recesses between the longitudinal support webs are completely filled by the sealing material as is known from EP-A-0 685 611. It has been shown for shock absorber pistons on which the collar-shaped seal is formed in the manner described in EP-A-682 190 that it is enough if, in addition to the peripheral web, the longitudinal support webs press into the material of the seal over only a portion of their height as well. On one hand, this results in an acceptable form-lock between the collar-shaped seal and the piston body, and indeed both in the longitudinal direction as well as in the peripheral direction. On the other hand, only moderate shaping forces on the sealing material result. The result is that not only do minimal pressure forces have to be applied, but material flow is also kept to a very minimum during the deformation, thus preventing a disadvantageous influence on the material structure for practical purposes. Even if the circular sleeve is deformed to such an extent that the notch-shaped recesses are completely filled in, the sealing material deforms in the perpendicular direction due to the motion of the piston body in the direction of the recess. This effects a favorable xe2x80x9cmaterial flowxe2x80x9d so that even in case of this deformation, a disadvantageous influence of the material structure is prevented. Since the notch-shaped recesses between the support webs are not completely filled in due to their being only partial pressed, enough free space remains into which the sealing material can give way when expansions occur due to temperature increases, while the piston is guided by the longitudinal support webs with no problem. It is a good idea here if the longitudinal webs extend up to the end of the piston body.
In another advantageous embodiment of the invention, it is provided that each of the edges of the collar-shaped seal extends beyond the end surface of the piston body associated with it. Since the collar-shaped seal is produced according to a known process from a circular sleeve, the phenomenon of xe2x80x9cback memoryxe2x80x9d of the sealant material described in EP-A-0 682 190 can be used to cause the edge around the inner diameter of the circular sleeve to pull inward after it is applied to the piston body, and to cause the edge of the collar-shaped seal produced from the external edge of the circular sleeve to move back outward and in this way to protrude above the rest of the peripheral surface of the collar-shaped seal as a lip seal. If the piston body is installed such that, when used as a shock absorber piston, the piston surface provided with the lip-shaped edge extending outward faces the pressurized side, i.e. the side subjected to the high load, and if the piston surface with the edge that springs back inward is located on the so-called suction side, this results in an improved seal of the piston in the shock absorber cylinder during a pressure load since the hydraulic fluid in the lip-shaped edge presses against the cylinder wall. When it springs back, i.e. for suction loads, the hydraulic fluid can then enter the intermediate space between the piston wall and the cylinder wall to some degree as a result of the minimal play between the two. The fluid can thus find its way up to the edge where the lip-shaped edge of the collar-shaped seal sits against the cylinder wall for the purposes of lubrication. Thus, acceptable lubrication is provided, preventing wear of the seal. Since the flow resistance due to this minimum gap between the sealing surface of the piston and the cylinder wall is considerably higher than the flow resistance through the flow channels in the piston body, practically no drop in performance results.
In an especially useful embodiment of the invention, it is provided that the longitudinal webs on the piston body run at an angle on the peripheral surface with respect to the piston axis. As a result of the slanted arrangement of the support webs, the advantage arises in that the entire surface of the piston sits on its seal as seen from the peripheral direction since each notch sitting between two longitudinal webs is overlapped when the longitudinal webs are slanted accordingly with respect to the piston axis. This provides an even contact pattern on the surface of the seal over the height of the piston, even if the sealant material should drop into the incompletely filled notch-shaped recesseso some degree. Since the mechanical load on the form-locked connection between the seal and the piston body occurs only in the direction of motion for practical purposes, the slanted arrangement of the support webs offers an additional component of support in the direction of motion.
Other embodiments of the invention are given in the subordinate claims.